Yes there is such a thing as spiritual communion. One who cannot recieve for one reason or another may make a spiritual communion. Suggested prayers for such can be found in most Catholic prayerbooks. As for the spiritual benefits, actual communion is the best way of receiving the graces of the sacrament but if one is unable to receive, such as a person in the hospital or in bed and communion cannot be brought to him or her, it is a laudable practice. While anyone, a priest or layman can assit someone who cannot receive communion either through reception of a consecrated host, or consecrated wine, one can encourage and even help someone to make a spiritual communion, by reminding them that they can, and if they are unsure how to do so by leading them in a suitable prayer to make a spiritual communion.
As for someone in mortal sin who is unable to confess his or her sins for a good reason, an act of perfect contrition, (sorrow for ones sins because they offend God) would suffice. It is also a good practice to in one’s daily prayer routine to at least daily express one’s sorrow for any sins we may commit throughout the day, asking God’s forgiveness, and for the graces to truly repent of our sins. While this does not remove the need for the sacrament of penance, (reconciliation) it does form good spiritual habits in our lives. Just as we can fall into habitual sins, we can also cultivate habitual grace in our lives. God will also not turn away from someone who makes a habitual effort to amend his or her life, and live according to His dictates. Just as one condition of the valid reception of forgiveness in the confessional is sorrow for ones sins, and the amendment to try and not sin in the future, God will not withold his forgiveness if we do not have access to a priest to recieve the sacramental absolution that he comissioned the Apostles with when Jesus told them “who’s sins you forgive, they are forgiven, who’s sins you retain they are retained.”